1. Field of the Invention
The present invention relates to biaxially oriented multilayer polypropylene films. More particularly, the invention relates to biaxially oriented multilayer polypropylene films having extremely slight stretching unevenness and being superior in transparency, see-through feeling, hot slip property, rigidity, heat resistance, durability of antistatic property and printability.
2. Description of the Related Art
Biaxially oriented polypropylene films are used in a wide variety of applications such as food packaging applications and textile packaging applications because of their superior optical characteristics such as transparency, see-through feeling and luster, mechanical characteristics such as tensile strength and rigidity, and moisture barrier properties. Biaxially oriented polypropylene films, however, are likely to have static electricity and may suffer from adhesion of dust to their surface or defective fixation of ink to their surface at the time of printing. As biaxially oriented polypropylene films having improved antistatic properties, films such as those shown below are known.
Japanese Patent Application Laid-Open (JP-A) No. 5-104688 discloses a biaxially oriented composite polypropylene film that comprises a base layer of polypropylene with a density of 0.9070 g/cm3 containing an antistatic agent and a polypropylene layer laminated on one or both sides of the base layer. Specifically, disclosed is a biaxially oriented composite film produced by using polypropylene homopolymer for forming both a base layer and a surface layer and stretching an intermediate laminate transversely at a stretch ratio of 10 at a stretching temperature of 166° C.
JP-A No. 5-112682 (also issued as U.S. Pat. No. 5,331,054) discloses a biaxially oriented multilayer polypropylene film that has a surface layer formed of a polypropylene composition which contains a propylene polymer with a molecular weight distribution of 3 or less obtained in the presence of a catalyst comprising a metallocene compound and alumoxane, and a propylene-based random copolymer with a molecular weight distribution of 3.5 to 10 obtained in the presence of a Ziegler type catalyst. Specifically, disclosed is a biaxially oriented multilayer polypropylene film which has a surface layer formed of a composition containing, as the propylene polymer obtained in the presence of a catalyst comprising a metallocene compound and alumoxane, (i) a propylene polymer whose DSC chart has twin peaks at 141.0° C. and 135.7° C., (ii) a propylene polymer whose DSC chart has twin peaks at 134.2° C. and 127.0° C., or (iii) a propylene polymer having a melting point of 135.2° C., the film being obtained by stretching an intermediate sheet laterally at 168° C.
JP-A No. 10-16153 discloses a laminated film that comprises: a biaxially oriented polyolefin film which comprises a propylene homopolymer and/or propylene-α-olefin copolymer having an isotactic pentad fraction of 0.83 to 0.93 and a surfactant incorporated therein, and a surface layer which comprises a propylene homopolymer and/or a propylene-α-olefin copolymer having a content of α-olefin other than propylene of 8 mole % or less and a polyolefin having a content of α-olefin having 4 or more carbon atoms of 15 mole % or more, the surface layer being laminated on at least one surface of the polyolefin film.
JP-A No. 2002-248719 discloses a biaxially oriented multilayer polypropylene film which comprises a substrate layer having a surface layer laminated on at least one surface thereof. The substrate layer comprises an antistatic agent and polypropylene which has a cold xylene-soluble fraction content of less than 1.5% by weight and a melt flow rate of 0.5 to 20 g/10 min. and in which a melting point (Tm, ° C.) determined on the basis of a DSC chart and a cold xylene-soluble fraction content (CXS, % by weight) satisfy the following formula (I):2(CXS)+152≦Tm≦2(CXS)+159  (I).The surface layer is formed of, polypropylene which has a cold xylene-soluble fraction content not less than 2% by weight and a melt flow rate of 0.5 to 20 g/10 min. and in which a melting point (Tm, ° C.) determined on the basis of a DSC chart and a cold xylene-soluble fraction content (CXS, % by weight) satisfy the following formula (II):2(CXS)+148≦Tm≦2(CXS)+155  (II).Specifically, disclosed is a multilayer biaxially oriented polypropylene film obtained by using a propylene-ethylene random copolymer having an ethylene content of 0.6% by weight and a melting point of 158.0° C. as a surface layer and stretching an intermediate film transversely at a stretching temperature of 162° C.